Glacial Periods in Earth’s History

Source: Steven, E. (2015)

Major Glaciations

• Huronian Glaciation (2.4 - 2.1 Ga): One of the earliest recorded glaciations.
• Cryogenian “Snowball Earth” (700 - 650 Ma): Period of global-scale glaciation.
• Phanerozoic Glaciations:
  • Andean/Saharan (450 - 420 Ma)
  • Karoo (360 - 260 Ma)
  • Cenozoic (34 Ma - Present)

Pleistocene Glaciation

• Cycles of glacial and interglacial periods driven by Milankovitch Cycles.
• Last Glacial Maximum (20 ka): Ice sheets covered ~30% of land.

Causes of Glaciation

• Tectonic activity (e.g., Himalayan uplift) increased erosion and removed CO₂, cooling the climate.
• Changes in ocean currents (e.g., the Antarctic Circumpolar Current) isolated polar regions, leading to glaciation.
• Milankovitch Cycles (changes in Earth’s orbit and tilt) regulate glacial-interglacial cycles.

How Glaciers Work

Source: Steven, E. (2015)

Types of Glaciers

• Continental Glaciers: Cover vast land areas (Antarctica, Greenland).
• Alpine Glaciers: Found in mountainous regions.

Glacier Formation & Movement

Source: Steven, E. (2015)

• Glacier: A mass of ice that moves under its own weight.
• Forms when accumulated snow compacts into firn, then glacial ice.
• Equilibrium Line: The boundary between the zone of accumulation and the zone of ablation.
• Glacial advance: More accumulation than melting.
• Glacial retreat: More melting than accumulation.
• Ice Flow Mechanisms:
  • Internal Flow: Ice deforms plastically under pressure.
  • Basal Sliding: If water is present at the base, ice slides over the bedrock.
  • Crevasses: Surface cracks due to differential movement.

Glacial Erosion

How Glaciers Erode

• Glacial ice alone does not erode effectively; embedded rock fragments act like sandpaper.
• Plucking: Ice pulls away chunks of bedrock.
• Abrasion: Rock fragments scrape and polish surfaces.

Alpine Glacial Features

• U-Shaped Valleys: Deep valleys with steep walls, carved by glaciers as they move.
• Arêtes: Sharp ridges between glacial valleys.
• Horns: Steep, pyramid-like peaks.
• Cirques: Bowl-shaped depressions where glaciers form.
• Hanging Valleys: Tributary valleys left at higher elevations.
• Truncated Spurs: Ridges cut off by glacier erosion.

Alpine Glacial Features

Source: Steven, E. (2015)

Continental Glacial Features

• Drumlins: Streamlined hills shaped by glaciers.
• Roches Moutonées: Asymmetrical rock features smoothed by ice.
• Glacial Striations: Grooves carved by moving glaciers.
• Moraines: Ridges of till deposited at glacier edges.
• Eskers: Sinuous ridges formed by subglacial rivers.
• Kettles & Kettle Lakes: Depressions formed by melting ice blocks.
• Outwash Plains (Sandurs): Extensive sediment deposits from glacial meltwater.

Continental Glacial Features

Source: https://geo.libretexts.org/

Glacial Striations

Source: https://viewer.gigamacro.com

Factors Affecting Erosion

• Ice thickness, slope, presence of rock fragments.

How Glaciers Deposit Material

• Till: Unsorted sediments deposited directly by glaciers.
  • Lodgement Till: Compacted beneath the glacier.
  • Ablation Till: Deposited from melting ice.
• Glaciofluvial Deposits: Sorted sediments deposited by meltwater.
• Glaciolacustrine Deposits: Sediments deposited in glacial lakes.
• Glaciomarine Deposits: Sediments deposited in the ocean from icebergs.

Glacial Lakes

• Tarns: Cirque lakes.
• Finger Lakes: Long, narrow lakes in glacial valleys.
• Moraine Lakes: Formed behind moraines.
• Kettle Lakes: Small depressions filled with meltwater.

Summary

• Glaciation has shaped Earth’s landscape through erosion and deposition.
• Glaciers are dynamic systems influenced by climate and topography.
• Understanding glacial processes helps reconstruct past climates and landscapes.